Processes and equipment for reducing gaseous emissions

The destruction and removal of pollutants occurring in exhalates is performed by adsorption, absorption and catalytic processes, by combustion or by specific chemical processes. The capture of harmful substances is often complicated on account of both the low concentrations of these substances in the gas and high flow rates. 

During the desulphurization of combustion products, absorption is the only method which is widely used in practice. Suspensions of lime, limestone, magnesium oxide or alkalies are used most frequently. 

Different types of absorbers are used for absorption. The most simple unit is an absorbing tower with sprayed liquid, with countercurrent motions of the gas and liquid. The efficiency depends on the size of drops produced and on the uniformity of their distribution in the tower. The absorber should be equipped with a drop separator. Low pressure losses and simple design are the advantage of this type of equipment. 

A fairly considerable hydraulic resistance and sensitivity to fluctuations of the gas flow rate are disadvantages of the foam absorbers. The disadvantage may be partially overcome by using plates with adjustable openings, 

Adsorption is the process of concentration of gases, vapours or liquids on the surface of pores of solid materials, as a result of adsorption forces. Two types of the adsorption forces may be considered — physical and chemical. When the substance adsorbed is bound to the surface of the adsorption agent by intermolecular forces (particularly dispersion forces), the adsorption is of a physical nature. Chemisorption occurs when these forces have the character of chemical interactions. The adsorption process depends on the temperature, pressure and chemical composition of the adsorbed gas and adsorbing material. The adsorption is always connected with a release of adsorption heat (and thus an appropriate level of cooling should be provided for large adsorption units). 

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